This invention relates to the production of low gloss thermoplastic resins, and more particularly to an improved method for formulating an additive which imparts low gloss properties.
The production of low gloss blends of polycarbonates and acrylonitrile-butadiene-styrene graft copolymers (hereinafter "ABS resins") is described in various publications and patents. A particularly useful additive is the reaction product of a polyepoxide with a polymer containing nitrile groups. U.S. Pat. No. 5,026,777, for example, discloses the incorporation of a polyepoxide into an alloy of ABS and polycarbonate in a single blending step. Copending, commonly owned application Ser. No. 08/110,142 describes an improved process in which a gel is first produced by the reaction of a polyepoxide with a polymer containing nitrile groups, typically a styrene-acrylonitrile copolymer (hereinafter sometimes "SAN copolymer"). Said gel is next dispersed in a first thermoplastic resin such as a polycarbonate to produce a concentrate which is then blended with further resin such as ABS resin or an ABS resin-polycarbonate mixture.
These operations are usually carried out in an extruder. The results include improvement in temperature control, minimized degradation and improved gel utilization.
Certain problems may be encountered in this process by reason of unfavorable viscosity properties of the materials employed. SAN copolymers, for example, have low viscosity which minimizes viscous heating during blending with the polyepoxide, making it difficult to reach a suitable reaction temperature. Upon reaction, however, viscosity builds to the point where passage through the extrusion die and formation of a strandable product are very difficult to attain.